I wouldn't rely on Defender by itself as primary. You can set it to do periodic scans even with another AV installed though. So as a secondary it's ok. I replaced Norton on all the PC's with Bitdefender free. Used same account for all, which supports up to 30 devices, and allows you to monitor all devices at once. I would rather have installed Eset Nod32, but they don't have a free version for PC. On the Android devices I replaced Norton with Eset mobile (which turns into free Eset antivirus (aka Nod32) after the 30 day trial ends). Eset scans all the files on your card/device, not just installed apks (android apps). Installing AV on mobile is controversial. Some say you don't need it. It's a known fact that malware always seems to get by Google's scanners on the Play store though. I don't want to take the chance as I do mobile banking, taxes, etc. Also, I torrent, and I also share/download files from websites/off internet, and I don't want my mobile device to be a vector that can infect our PC's, or other network IoT/smart devices when I connect to them. There is a lot of Android malware out there. It's easy to get infected. I run AV/anti-malware on mobile just to be safe.
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